1. Field of the Invention
The present invention relates to a magnetic head position controller in a magnetic recording and reproducing apparatus, such as video tape recorders and digital tape recorders, in which a movable magnetic head mounted on a rotating drum is controlled in its position along the length of the magnetic tape by an actuator.
2. Description of the Prior Art
In magnetic recording and reproducing apparatuses such as VTR, reproducing data recorded on the magnetic tape requires that the magnetic head follow the recording track accurately. For making the magnetic head follow the recording track accurately, conventional improved VTRs control the position of the magnetic head for correct tracking.
For this purpose, the magnetic head is mounted on the rotating drum in such a way that it is deflected by an actuator in the tracking direction in order to prevent deviations from the correct track. In a special replay operation such as a fast replay, the magnetic head is moved in a large span toward the tracking direction to perform various fast and synchronous replay in good condition.
FIG. 47 shows an external view of a piezo-electric bimorph element 500. A pair of piezo-electric bimorph elements 500 are provided symmetrically on the diagonally opposite sides of the rotating drum 520 and each has a magnetic head H mounted thereon.
FIG. 48 shows an improved version of the bimorph element 500 shown in FIG. 47. The bimorph elements 500 extend in a semicircular arc along the circumference of the rotating drum 520 so that the effective length of each bimorph element 500 will be longer than that of FIG. 47.
FIG. 49 also shows an improved version of the bimorph element 500 illustrated in FIG. 47. The bimorph elements 500 are arranged parallel to the chord of the rotating drum 520 so that the effective length of each bimorph element 500 is longer than that of FIG. 47.
FIG. 50 shows the state of the bimorph element 500 and the magnetic head H when the bimorph element 500 drives the head H. When the bimorph element 500 having the effective length equal to radius R is bent through an angle .theta., the magnetic head H moves the distance .xi..
FIG. 51 shows the geometric relationship between the inclination of the magnetic head H (.theta. in FIG. 50) and the effective length of the bimorph when the bimorph 500 is driven. The abscissa represents the effective length of the bimorph and the ordinate represents the inclination of the magnetic head H.
FIG. 52 shows one example frequency characteristic of the piezo-electric bimorph element 500.
The control of the position of the magnetic head by bending the bimorph element to perform correct tracking should be accurately carried out while at the same time it is also necessary to precisely control damping of mechanical resonance of a spring mass system for the magnetic head and the drive actuator.
FIG. 53 shows a block diagram of the magnetic head driving apparatus using the conventional bimorph actuator disclosed in Japanese Patent Preliminary Publication No. Showa 52-117107.
The magnetic head driving apparatus using the conventional bimorph actuator has the following constitutional elements: the piezo-electric bimorph element 500 which performs a bending operation according to the applied voltage to perform a desired tracking control by moving the magnetic head H in a direction perpendicular to the direction of the tape travel; a sensor 501 made up of a piezo-voltage generator formed as part of the piezo-electric bimorph element 500; a high-input impedance amplifier 502 which amplifies the detected voltage while applying practically no load to the sensor 501; an adder 503 that adds an output from the high-impedance amplifier 502 and an output from a potentiometer 509 described later; a differentiator 504 to differentiate the output from the adder 503; a low-pass filter 505 having a cutoff frequency that is so selected as to attenuate only the signal which contributes to the secondary resonance characteristic and high-order resonance characteristic; a phase advance circuit 506 to compensate for a phase delay of the output from the low-pass filter; a gain variable amplifier 507 to variably reverse-amplify the output from the phase advance circuit 506; an adder 508 that adds an output signal from a frequency compensator 511 described later and an output signal from the gain variable amplifier 507; a potentiometer 509 to which the output signal from the adder 508 is supplied; and a drive amplifier 510 which amplifies the output signal from the adder 508 and applies a desired drive voltage to the bimorph actuator 500.
The magnetic head driving apparatus also includes; a video signal processing circuit 514 which video-processes the output from the magnetic head H supported at the free end of the cantilevered piezo-electric bimorph element 500; a head position regulating circuit 513 which outputs a tracking compensation signal based on the output signal of the magnetic head H to form a wobbling servo system; a frequency compensator 511 that compensates the frequency in response to the output signals from the head position regulating circuit 513 and from a convertor reset signal generator 512 described later; and a convertor reset signal generator 512 which generates a reset signal to be applied to a deflectable support arm, i.e., the prizo-electric bimorph element 500, in order to selectively reset the magnetic head H to the initial position of tracking.
Now, the operation of the magnetic head driving apparatus using the conventional bimorph actuator will be described.
The sensor 501 formed integral with the piezo-electric bimorph element 500 generates a signal representing the instantaneous deflection position of the magnetic head H.
The output signal lags the signal for driving the piezo-electric bimorph element 500 by 90 degrees in phase.
This output signal is supplied to the high input impedance amplifier 502. The reason why the amplifier 502 is of a high input impedance is that since the sensor 501 is equivalent to a capacitor connected in series with a voltage source, the sensor 501 must have a small electrical load to achieve effective coupling of a low-frequency signal from the sensor 501.
The output of the high impedance amplifier 502 is sent to the adder 503, which also receives at another input an output signal from the potentiometer 509 described later. The output signal of the adder 503 is given to the differentiator 504, which differentiates the head position signal from the sensor 501 to convert the head position signal representing the instantaneous head position into a signal representing the instantaneous head speed.
Since the differentiator 504 has a frequency characteristic similar to that of the high-pass filter, the signal that has passed through it is advanced in phase. The head speed signal produced by the differentiator 504 is supplied to the low-pass filter 505, whose cutoff frequency is so selected as to virtually attenuate a signal that contributes to the secondary resonance characteristic and high-order resonance characteristic of the bimorph element 500.
The low-pass filter 505 delays the signal in phase that passes through it and, to compensate for the total phase delay, caused by the low-pass filter, of a signal near the resonance position, a phase advance circuit 506 is provided. The phase advance circuit 506 shifts the phase of the signal component whose frequency is close to the resonance point of the bimorph element 500 so that the signal, when output from the phase advance circuit 506, has a phase of zero degree.
The output signal of the phase advance circuit 506 is sent to the gain variable amplifier 507 where it is inverted and then sent to the adder 508, which adds an output signal from a frequency compensator 511 described later to the inverted signal of the amplifier 507 to attenuate the resonance oscillation of the bimorph element.
The output of the adder 508 is amplified by the drive amplifier 510 and is output as a deflection drive signal for the bimorph element 500. The gain variable amplifier 507 is so constructed as to be able to adjust the gain to cope with variations in characteristic of the bimorph element 500.
A signal component close to an antiresonance point is effectively adjusted to zero by partially coupling the drive signal supplied to the bimorph element 500. The deflection signal of the adder 508 is given to the potentiometer 509, whose output is supplied to the other input terminal of the adder 503 where it is added with the deflection position signal, which is entered from the high impedance amplifier 502 and detected by the sensor 501.
The phase of the deflection signal is shifted 180 degrees as it is detected by the sensor 501 through the bimorph element 500, so that the frequency component of the deflection signal near the antiresonance point is zero-adjusted by the adder 503 to stabilize the loop at the frequency near the antiresonance point.
In this way, the bimorph element 500 is damped to enable a stable tracking control.
However, since the bimorph element 500 in FIG. 53 is mounted on the rotating drum 520, as explained in FIG. 47, a large amplitude operation as during the special replay operation of the VTR causes the magnetic head H to be displaced upward in FIG. 50 and inclined by an angle .theta..
This degrades the contact condition between the magnetic head H and the magnetic tape, an important factor that deteriorates the high frequency characteristic of the recording and reproducing signal.
To make an improvement on this drawback, it is proposed to increase the effective length of the bimorph element 500 as previously shown in FIGS. 48 and 49. While the longer effective length reduces the head inclination in FIG. 50, the resonance frequency and antiresonance frequency of FIG. 52 shift toward the lower side.
General bimorph elements have a characteristic in which the signal in a frequency band higher than the first-order resonance frequency is shifted in phase by 180 degrees. Hence, in the tracking system using a movable head, the control frequency band must be set sufficiently lower than the resonance frequency.
If the first-order resonance frequency and the second-order resonance frequency or the antiresonance frequency are sufficiently apart from each other, the phase advance compensation makes it possible to provide the control frequency band between the first-order and the second-order resonance frequency or the antiresonance frequency.
However, the bimorph element 500 with too long an effective length cannot have a sufficiently large control frequency band, making it difficult to form a control system that can reliably follow the track curve on the magnetic tape.
Although it is possible to reduce the resonance peak gain of the bimorph element 500 to a certain extent by using the differential circuit described earlier, the gain of the damping loop cannot be set sufficiently large because the differential operation increases the noise contained in the deflection position signal.
As mentioned above, in the magnetic head driving apparatus using the bimorph element 500, providing a good contact between the magnetic head H and the magnetic tape during the tracking operation (i.e., to reduce the head inclination) is not consistent with shifting the mechanical resonance of the bimorph element 500 toward higher frequency or reducing the resonance peak gain. These two conflicting requirements are difficult to meet at the same time.
In the damping control circuit using a differential circuit, there is a limit to the performance improvement because the noise of the position sensor increases with the performance enhancement.
In addition, there are mechanical restrictions. Since the position signal must be taken out when forming a damping loop, it is necessary to install a printed circuit card for damping in the rotating drum or the position signal must be taken out by a slip ring.
The damping method employed in the conventional magnetic head driving actuator differentiates the position signal of the actuator to find the actuator speed, as described above. This also increases noise contained in the position signal, making it difficult to obtain a good damping performance.
The conventional magnetic head driving actuator has a drawback of mechanically complicated construction. That is, the position sensor is necessary to pick up the above-mentioned position signal; and when the position signal is to be taken out of the rotating drum, it is necessary to increase the number of slip ring channels.
Furthermore, since the resonance frequency depends on a mechanical spring and mass system, a substantial improvement of the response characteristic of the actuator is so far not possible.
Moreover, the rigidity against external load disturbance (periodical impact of the tape and head) depends only on the mechanical rigidity of the spring and mass system and on the magnetic damper effect of the magnetic circuit system, so that the mechanical design requires special considerations which in turn provide constraints on the actuator design.